(1) Field of the Invention
The present invention relates to the field of rotorcraft, and it relates more particularly to arrangements associated with the antennas of radioaltimeters fitted to rotorcraft.
(2) Description of Related Art
Rotorcraft are conventionally fitted with a radioaltimeter providing data about the height of the rotorcraft above the ground. A radioaltimeter uses a pair of antennas, including a transmit antenna transmitting a radio signal as pulses or continuously, and a receive antenna picking up said radio signal after it has been reflected on the ground.
The height of the rotorcraft above the ground is determined by a computer of the radioaltimeter by taking the time that elapses between the transmit antenna transmitting the radio signal and the receive antenna receiving the radio signal, and dividing the time by the propagation speed of an electromagnetic wave in air.
Radioaltimeter antennas are positioned on board a rotorcraft so as to avoid the radio signal being reflected against the structure of the helicopter. That is why it is conventional practice for the antennas to be installed on the underside of the rotorcraft, with the underside location of the antennas under the rotorcraft naturally being as considered relative to the position of the rotorcraft when on the ground. Furthermore, the antennas of the radioaltimeter are commonly protected from the outside environment of the rotorcraft, in particular from bad weather, by a radome that is typically arranged as a cupola that is transparent to radiowaves.
Nevertheless, rotorcraft are sometimes used for specific flight missions that involve transporting a slung load. In this context, there arises the problem of potential interference between the load being transported by sling and the radio signals exchanged between the antennas of the radioaltimeter.
The load being transported by sling swings under the rotorcraft, thereby making it possible while the rotorcraft is in motion, for the load to intrude into the radioaltimeter's field of view as defined by the antennas. In this context, such an intrusion runs the risk of the radio signals that are being exchanged between the antennas being reflected on the load instead of on the ground, and consequently making the pertinence of the height supplied by the radioaltimeter doubtful.
Such a risk of the load intruding into the field of view of the antennas increases when the location of the antennas on board the rotorcraft is at a short distance from the sling equipment, as happens in particular with a rotorcraft of moderate size.
Such a risk of the load intruding into the field of view of the antennas has been known for a long time, and one solution that is conventionally used for avoiding any risk of interference between the load being transported by sling and the radio signals exchanged between the antennas consists in installing “horn” antennas on board the rotorcraft so as to define a narrow field of view for the radioaltimeter.
Nevertheless, such a solution is not satisfactory, since the ability to measure the height of the rotorcraft above the ground becomes limited when the rotorcraft is performing turns that are strongly banked, in particular in roll, and/or when the rotorcraft is overflying terrain that presents relief that is complex and varied.
Consequently, given the occasional nature of rotorcraft performing flight missions that involve transporting a slung load, it is conventional to mount antennas on board rotorcraft that are of plane configuration so as to obtain an optimized field of view for the radioaltimeter. Such provisions make it possible in particular to provide the pilot of the rotorcraft with reliable information about the height of the rotorcraft above the ground for most flight missions of a rotorcraft, except when transporting a slung load.
Consequently, the plane antennas of the radioaltimeter advantageously identify an optimized field of view for the radioaltimeter, the radioaltimeter commonly being taken out of operation by the crew of the rotorcraft when transporting a slung load.
Another solution consists in replacing the antennas of the radioaltimeter from a set of interchangeable antennas, depending on the flight mission of the rotorcraft. Nevertheless, such operations are expensive and difficult to perform, and in practice they are little used.
In the general field of transport, numerous solutions have been proposed for mounting radio antennas on a movable structure so as to present a field of view that is adapted to requirements, in particular by adjusting the direction in which the antennas point or by special arrangements for the radome. By way of example, reference may be made to the following documents: U.S. Pat. No. 6,452,567 (Harris Broadband Wireless Access, Inc.); U.S. Pat. No. 7,030,834 (Harris Corp.); U.S. Pat. No. 7,088,308 (Harris Corp.); EP 1 907 882 (Robert Bosch GmbH); and WO 2007/118211 (Andrew Corp.).
Document US 2005/253750 (Honeywell International, Inc.) describes a radar altimeter for a vehicle operating with a suspended load. In order to maintain a desired altitude in flight while not moving forwards, a vehicle such as a helicopter needs to detect an accurate altitude above the level ground (AGL). The radar altimeter delivers radiofrequency (RF) pulses at regular intervals to an antenna that transmits beams to the ground. The beams are then reflected and picked up by an antenna of the radar altimeter. Load profile processing limits the altitude sensitivity for certain distances between the radar altimeter and the suspended load so as to reduce the risk of processing signals reflected by the suspended load. The radar altimeter distinguishes between radar reflections coming from the suspended load and reflections coming from the ground on the basis of these reflections having distinct signatures. Logic gates for tracking altitude and for acquiring altitude act as logic switches that enable only certain examples of reflections to be processed.
Document XP 035317934 entitled: “System architecture of HALAS-a helicopter slung load stabilization and positioning system”, by D. Nonnenmacher et al., was published by Deutsches Zentrum für Luft and Raumfahrt, Germany, on Dec. 10, 2013. That document mentions varying the altitude of a helicopter for indirectly damping pendular movements of a load suspended under the helicopter.
Document U.S. Pat. No. 3,088,109 (Lab for Electronics, Inc.) describes a Doppler navigation radar for accurately specifying the speed components of a helicopter, including while hovering. The energy returned by various transmit beams is received separately and the respective frequency spectra are converted into a frequency spectrum scope close to a tracking frequency.
Document EP 1 933 163 (Franco Baldi) describes detecting an obstacle by aiming and focusing transmitted waves, for various types of moving body including cars, trains, floating vessels, aircraft, missiles, or automatic aiming systems. The power of the signals transmitted or received by an obstacle detector is amplified. Obstacle detection makes provisions for various antenna output lens shapes and for the possibility of discriminating in different directions. Various lens shapes are described.
Document JP 3 755 225 (Kokusai Electric Co. Ltd.) describes a radioaltimeter mounted on a helicopter. In order to avoid zero correction of an altimeter measurement, a time interval is taken into account between transmitting and receiving waves.